Measuring fractional cover and leaf area index in arid ecosystems: Digitalcamera, radiation transmittance, and laser altimetry methods

Field measurement of shrubland ecological properties is important for both site monitoring and validation of remote sensing information. During the Ma y 1997 NASA Earth Observing System Jornada Prototype Validation Exercise, w e calculated plot-level plant area index leaf area index, total fractional cover, and green fractional cover with data from four instruments: (1) a Dy cam Agricultural Digital Camera (ADC), (2) a LI-COR LAI-2000 plant canopy a nalyser, (3) a Decagon sunfleck Ceptometer, and (4) a laser altimeter. Esti mates from the LAI-2000 and Ceptometer were very similar (plant area index 0.3, leaf area index 0.22, total fractional cover 0.19, green fractional co ver 0.14), while the ADC produced values 5% to 10% higher. Laser altimeter values, depending on the height cutoff used to establish total fractional c over were either higher or lower than the other instruments' values: a 10-c m cutoff produced values similar to 80% higher, while a 20-cm cutoff produc ed values similar to 30% lower The LAI-2000 and Ceptometer are designed to operate in hamogenons canopies, not the sparse and irregular vegetation fou nd at Jornada. Thus, these instruments were primarily useful for relative w ithin-site plant area index monitoring. Calculation of some parameters requ ired destructive sampling, a relatively slow and labor-intensive activity t hat limits spatial and temporal applicability. Validation/monitoring campai gns therefore should be guided by consideration of the amount of time and r esources required to obtain measurements of the desired variables. Our resu lts suggest that the ADC is both efficient and accurate for long-term or la rge-scale monitoring of arid ecosystems. (C) Elsevier Science Inc., 2000.